They had Leeta, Kai Winn and Kasidy as major recurring characters and they could have used Keiko morw, it's not like Kira was the only female left on the show. TNG had Marina Sirtis as the only female main cast member during season 2, it happened before.
Or they could have introduced a new female main character and have the male Dax be recurring.
At the time, it was said that they had to replace Jadzia with another woman instead of perhaps a man or not replacing her and making her recurring because it would have left Kira the only female lead. I agree with you, though. There were several female secondary characters who could have been 'elevated' and given better storylines. DS9 was always so good at developing their secondary characters that I think it would have worked just fine.
(BTW, Marina Sirtis was not the only female lead during TNGS2. Diana Muldar played Dr. Pulaski and was a lead character
I think a male Dax would have been more interesting, it would have been great to see what changing gender does to a joined Trill. How does a straight male host deal with memories of being in love with men, especially if the former husband is still around? What about Bashir? What would he feel if the male Dax still has feelings for him and is similar in personality to Jadzia Dax? What if Bashir recognized some of Jadzia's quirks that he thought of as cute or lovable before, woult the person be more important of the newly attached penis?
Things like that were touched on a little bit during the Odan episode of TNG and the Lenara Khan DS9 episode but it would have been different with major characters over an entire season or at least part of it, the most important part would have been that it involves male characters, the "gay stuff" in both the Odan and Khan episodes was about women.
I agree that it would have been interesting, but they would have never gone there. There was obviously going to be some tension between Worf and NewDax. But if NewDax had been a man, the writers would have completely ignored any of that. Yes, there was TNG's "The Host" and "Rejoined" but people are always more comfortable with female same-sex situations than with male. And I just can't imagine that homosexuality is something that's even remotely considered amongst Klingon men.